1. Field of the Invention
Embodiments discussed herein relate to a unit pixel of an image sensor and a photo detector of the unit pixel, and particularly, to a unit pixel of an image sensor and a photo detector using the unit pixel capable of adjusting a light detection sensitivity of a light-absorbing part by adjusting a voltage applied to a gate functioning as the light-absorbing part.
2. Description of the Related Art
An image sensor is configured to convert an optical signal to an electrical image signal. When light is irradiated to a light-absorbing part in a unit pixel of an image sensor, the image sensor detects the light incident to each unit pixel and the amount thereof, converts an optical signal to generate an electrical signal to, and transmits the electrical signal to an analog and digital circuit so as to form an image.
When light is incident to the light-absorbing part of the unit pixel of the image sensor, one electron-hole pair (EHP) is generated for one incident photon, and the generated electrons and holes are accumulated in a photo diode, which is the light-absorbing part.
The maximum accumulation electrostatic capacity of the photo diode is proportional to a photo detection area of the photo diode. Particularly, in the case of a complementary metal oxide semiconductor (CMOS) type image sensor, the area in which the accompanying transistors are arranged is relatively larger than that of a charge coupled device (CCD) type image sensor and thus, there is a physical limit on increasing the photo detection area. Moreover, the photo diode, which is commonly used as the light-absorbing part of the image sensor, has relatively small electrostatic capacity and thus, the photo diode is easily saturated with a small amount of light so that it is difficult to segment the signals in an analogue form.
Accordingly, the unit pixel of the CMOS image sensor requires a relatively long photoelectric charge accumulation time in order to generate a minimum electric charge for signal processing through the limited photo detection area. Therefore, it may not be easy to manufacture a high-density/high-speed frame image sensor by using the unit pixel having such a light-absorbing part.
In order to overcome such limits in the conventional image sensor, U.S. Pat. No. 8,569,806 B2 (“UNIT PIXEL OF IMAGE SENSOR AND PHOTO DETECTOR THEREOF”), published on Sep. 2, 2011 by the applicant of the present application, discloses an a unit pixel of an image sensor that changes a threshold voltage of a channel using tunneling of electric charge from a floating gate to a source electrode or a drain electrode. The specification of the present application incorporates the techniques disclosed in the entire specification of U.S. Pat. No. 8,569,806 B2 by reference.
However, in a photo detector of U.S. Pat. No. 8,569,806 B2, a threshold voltage for photoelectric conversion is determined depending on a concentration and polarity of an ion doped on a floating gate and a well for device isolation in the initial manufacturing process and accordingly, a sensitivity value of the image sensor is also determined depending on the concentration and polarity of the ion doped during manufacturing processes.
Eventually, in U.S. Pat. No. 8,569,806 B2, magnitudes and sensitivity values of a photoelectric current and a dark electric current in the unit pixel are mainly determined depending on a predetermined doping concentration of the floating gate in the manufacturing processes and thus, there is a problem in that a repetitive manufacturing procedure needs to be performed several times until an optimal threshold voltage condition suitable for the required specifications is satisfied.